The invention relates to a method and apparatus for early fire detection.
Automatic fire alarms are used for the timely detecting of fires, and thus for the protection of human life and property. They have become an integral part of protection concepts and safety systems, which are no longer designed without them. In particular, inventions in the domain of smoke detection made effective early warning possible. The installation of millions of automatic fire alarms all over the world proves the success of the products that have originated in these inventions. The extreme sensitivity of modern smoke detectors facilitates alarm at an early stage so that in most cases appropriate and timely measures can be introduced. However, high sensitivity to smoke conditions has gone hand in hand with an increase in the probability of erroneous triggering (false alarm) by deceptive phenomena that simulate fire. As a consequence of the increasing numbers of fire alarm systems, the increasing numbers of false alarms has created a problem that has become a steadily growing threat to the effectiveness and acceptance of this technology.
The manufacturers of automatic fire alarm systems have recognized this danger, and for years have been endeavoring to meet this threat through improvement on known detection principles and through the use of improved signal processing. The goal of all improvements is to reduce the number of false alarms without, at the same time, giving up the advantage of early warning, i.e., reducing the sensitivity.
One possibility for reducing false alarms consists of examining multiple fire related criteria for the triggering of an alarm, i.e., "multi-criteria alarms." Such an alarm will be triggered only if multiple detected fire related criteria simultaneously exceed preset values. In this way, the deceptive characteristic quantities that affect some of the sensors used are eliminated. One disadvantage of these methods is the possibility that genuine fires may not be detected because one of the sensors used does not produce a signal. This can happen when one of the sensors is defective or when one of the sensors is not giving off any signal because the fire characteristic quantity to which it reacts is not present.
Other methods endeavor, through the use of special algorithms, to interpret the sensor signals in such manner that only fire-specific processes lead to an alarm. Fire specific data are stored as reference values and are continuously compared with the sensor data. Through the use of correlation methods, the probability of whether a true fire or an error exists can be computed for the sensor signals. Although considerable progress was achieved with both methods, the false-alarm problem cannot by any means be considered solved. Another objection is that the results to date were achieved only with the use of considerable electronic complexity so that the cost/benefit ratio was not very beneficial. However, there is no doubt that a fire sensor which because of its specific capabilities reacts only to genuine characteristic fire parameters and not to false criteria (error quantities), represents the best solution to be sought.
When we study the characteristic criteria that occur when there is a fire, we note that in addition to the occurrence of combustion products, such as gases or aerosols, an increase in temperature can always be recorded. This means that there is no property-damaging fire without a heat source, whether as initiating factor or through the independent action of the fire itself. Sooner or later, every fire leads to an increase in ambient temperature. For this reason, the oldest and still most frequently used fire alarms are temperature detectors. However, in these alarms, as a rule, the alarm is triggered only when the ambient temperature climbs above 60.degree. C. By then, the fire has already spread considerably, and therefore we cannot characterize this type of alarm a genuine early warning type.
Efforts to increase the sensitivity of heat alarms led to the development of heat differential alarms, which sense only the speed with which the temperature rises. But even with this technology, the early stage of a fire, which as a rule is a smoldering fire, cannot be satisfactorily detected. On the other hand, it would not be appropriate further to lower the detection threshold of heat alarms, because, as research has shown, this would lead to frequent false alarms.
Another detection method consists of interpreting heat radiation from an active flame as a fire criterion. In the case of a naked fire, which has no preceding smoldering phase, such as for example all liquid fires, such alarms are the fastest detectors. To render them insensitive to error quantities, the always-present flickering of a flame is used. This radiation fluctuation has fire-specific qualities and can be used for the differentiation of error quantities. Through the use of appropriate filters, it has become possible to exclude the effects of outside sources of light.
However, these alarms also have the aforementioned disadvantage of being unable to detect the smoldering phase of a fire. In addition, flame alarms must be in direct optical contact with the source of light; that is, they must "see" the flame. It has already been indicated that the phenomenon or characteristic fire quantity "temperature" is a feature of each and every fire. An increase in the temperature of the fire material expresses itself on the one hand in the emission of heat radiation; on the other, it leads to an increase in the air temperature around the area of the fire. The consequence is a heat convection current that on the one hand conveys the combustion products to the ceiling and on the other hand leads to temperature fluctuations in the vicinity of that point. If this effect is exploited for early fire detection, however, false alarms caused by error quantities must be expected, since heat sources not ascribable to a fire (heaters, gas burners, etc.) can lead to temperature fluctuations in the ceiling area.